Typical and perfect use: how to maximise effectiveness of the pill

The oral contraceptive pill is the most common form of prescribed contraception in the UK. Despite this fact, many pill users are still not clued up on the best way to take it, and how it can be best used to prevent pregnancy.

There are two main groups of oral contraceptive pill: the progesterone only pill (POP or mini pill) and the combined oral contraceptive pill (COCP). Both pill types work in slightly different ways, and as such have different instructions on how they are taken. Typically, the mini pill is taken continuously, whereas the combined contraceptive pill traditionally has a seven day, pill-free break after every 21 days.

As suggested by its name, the POP only contains one hormone — progesterone. The COCP contains two hormones, both progesterone and oestrogen. Different pills suit different people and your doctor can help you decide which one is best for you.

Certain medications (both prescribed and over-the-counter) can interact with both the COCP and the POP, making them less effective. Make sure your GP knows you are using the pill as contraception if they are prescribing you medication, especially if the pill was obtained from a family planning clinic as it may not be on your records. The most common supplement that interacts with the pill (and needs to be avoided) is St John’s Wort, a remedy often bought to help with low mood or anxiety.

How effective is the pill for contraception?

To measure the efficacy of a form of contraception, data looks at women using the same form of contraception over the course of a year and the chance that they will experience an unintended pregnancy in this time.

If a woman uses no form of contraception for a year, the chance of pregnancy is 15%. This means that for every 100 women having intercourse over a year, 15 will get pregnant in that time.1

The information for types of contraception looks at rates for perfect use and typical use. Perfect use is simply that: following every instruction to the letter. Every pill is remembered, taken at the same time every day and never missed. In reality, very few women manage to take it perfectly.

Typical use is a bit more complicated. This includes the rates of unintended pregnancy in all people taking the medication. In reality, this includes women that don’t take it correctly and those who forget to take it occasionally, or even those who leave it in the drawer permanently. As a result, typical use can overestimate the risks for women that take the contraceptive method reliably.2

For both the mini pill and the combined pill, if taken perfectly, the chance of a pregnancy is 0.3%. So for every 330 women that take the mini or combined pill perfectly for a year, one of these will become pregnant.

However, if taken typically, the chance of an unintended pregnancy for both the mini pill and the combined pill is 9%. That is, for every 100 women taking it typically, nine will fall pregnant within a year. This is quite a high rate, but taking it typically is not necessarily how most women will take it.3

For most women, the true effectiveness of the pill is most likely to be somewhere in the middle of these statistics (i.e. very occasional slip-ups only). This is why when looking at NHS information, it is quoted that the COCP and the POP are effective more than 99% of the time if taken properly. So for every 100 women using either pill type reliably for a year, less than one will get pregnant. This in real terms is a much more reassuring number. Few women take the pill perfectly, but many take it reliably.

Missed pills and mistakes occasionally will happen. The important thing is to follow missed pill rules and seek advice for emergency contraception immediately if needed.

Progesterone only pill

The POP works in several ways, including stopping release of the egg from the ovary (ovulation) in the majority of women. It alters the lining of the womb (uterus), and alters the cervical mucus, making a much more hostile environment. As a result, sperm is much less likely to reach the egg, massively reducing the risk of a resulting pregnancy.

Instructions on how to initially start the pill can differ depending on where in the menstrual cycle you are. Your doctor will give you instructions specific to your situation.

What is the most effective way to take the POP?

Ideally, the mini pill should be taken at the same time of day, every day, without missing a dose. With the more traditional types (such as Micronor and Noriday that contain a type of progesterone called norethisterone), there is only a three hour window to take the pill in, if you miss your normal time. If taken later than that, it counts as a missed pill, increasing the chances of the contraceptive method failing. Why only a three hour window? Micronor has been shown to inhibit ovulation in around 50% of users. Therefore, these older-style pills rely mainly on changes to cervical mucus and the lining of the womb to be effective. These can change pretty quickly, hence the need to take the pill in a short, three-hour safety window.

The newer formulations of the POP, such as Cerazette (which contains the progesterone desogestrel), have a much more forgiving 12 hour window to take the pill in, making them increasingly popular.4 This is because, unlike the older-style POPs, Cerazette has been shown to inhibit ovulation in 97% of people taking it.

Realistically, most women will miss a pill at some point, so it is important to know what to do. If you have missed your mini pill, take it as soon as you remember, and take the next pill at the normal time. Make sure however that you use alternative barrier contraception (such as condoms) for the next 48 hours as you will not be protected from pregnancy.

If you miss a pill but have sexual intercourse regardless, go and see you doctor or pharmacist for emergency contraception.

If you vomit within two hours of taking the pill, take another as soon as you can. If you continue to vomit, use another form of contraception, for example condoms, for the next 48 hours.5

Combined oral contraceptive pill

The combined pill contains two hormones, an oestrogen and a progesterone. Most types give a standard dose of oestrogen and progesterone for 21 days, and then you take a pill-free break for seven days (or seven dummy pills may be included to make it easier to remember when to start the next strip).

The combined pill works by stopping ovulation. It also affects cervical mucus and the lining of the womb in a similar way to the mini pill. The seven day pill break causes a drop in hormones, and the lining of the womb (endometrium) breaks down, avoiding pregnancy.

How to take the combined pill perfectly

Ideally, the COCP should be taken at around the same time every day. In practice however, as long as the pill is taken no more than 24 hours late this should be okay, but it is best to err on the side of caution. If you miss a pill, you need to use other contraception, for example a condom, for seven days afterwards. The main reason it is advised to take the pill at the same time each day is to aid compliance by getting the person taking it into a routine and also to keep the hormone levels at a relatively steady level. In addition, the studies carried out on safety data have generally been done on people who have been advised to take the pill at the same time each day, and as such may not be accurate statistical representations of people taking them irregularly.

If you vomit within two hours of taking the COCP, or within four hours of taking Qlaira and Zoely, take another tablet as soon as you can. However, if you continue to vomit or have diarrhoea, you need to count each day as a missed pill and follow the above missed pill advice.6

If you miss a combined pill and have sexual intercourse, what you need to do depends on where in the pack the missed pill is and how many pills have not been taken. The leaflet accompanying the pill will give instructions on what to do, but you should see your doctor or pharmacist for advice and use alternative contraception for seven days. Depending on when the pill was missed and the timing of intercourse, you may need to go and get emergency contraception.

Who does the pill suit best?

The pill suits women who don’t have any problems remembering to take it every day (one good way is to set a reminder on your phone).

If this is something you may find hard then other forms of longer acting contraception may be a better choice for you. One type of contraception does not suit everyone, the key to success is finding one that is safe and acceptable to you, but also suits your lifestyle as well.

Realistically, most women will forget to take a pill at the same time every day once in a while, or for whatever reason won’t be able to take one on a particular day (for instance leaving the packet at home). While this is not ideal it is important to remember that this doesn’t mean you will definitely get pregnant, or that you should feel guilty. It just means that it is important to understand what this means for your body, and what you should do next to prevent pregnancy.

Jennifer is a General Practitioner, medical writer, parent, and founder of the Grace Kelly Ladybird Trust, registered charity for childhood cancer awareness and research (www.gracekellyladybird.co.uk). She also has a particular interest in women’s and children’s health, and enjoys medical writing, particularly helping make medical information easily accessible to those who want to find out more.